The present invention relates generally to the combustion of particulate solid fuel in combustors provided with vortex generating means for increasing the combustion efficiency and residence time of the fuel within the combustor, and more particularly, to a fluidized bed combustor provided with multistage vortex-generating systems in the freeboard which significantly increases the combustion efficiency of fluidized bed combustors.
Fluidized bed combustion systems are becoming of increasing importance in industrial and utility type applications because of their relatively high combustion efficiencies. In a typical fluidized bed, a fluidization zone is provided in which a combustible carbon-containing material and a granular material such as a sulfur scavenger, sand, or the like are fluidized by a stream of combustion supporting medium, usually air, passing through a distributor underlying the bed. The fluidization causes significant and turbulent mixing of the particulates in the air to provide an efficient combustion process. While fluidized bed combustion processes are relatively efficient, several problems or drawbacks are inherent in their use. For example, one of the problems which detracts from the low combustion efficiency is due to the passage of unburned solid particulates from the combustor by being entrained or elutriated in the gaseous products ascending from the fluidized bed.
One of the efforts employed to increase the combustor efficiency and overcome this problem is to provide for the collection and recycling of the unburned particulates by placing various arrays of cyclones or other gas-solid separators externally of the combustor and then returning the particulate material to the fluidized bed. Another such efforts is the use of a higher freeboard or high space above the fluidized bed. Generally, this freeboard in a fluidized bed combustor functions as a settling chamber or zone for the solid particulates elutriated or entrained in the gaseous combustion products emanating from the fluidized bed. The freeboard is also often of a sufficiently high temperature to provide for substantial combustion of unburned carbon-containing solids and combustibles in the gases. Normally, the height of the freeboard in a fluidized bed combustor was required to correspond to the so-called "transported disengaging height (TDH)" which is defined as the height of the freeboard where the concentration of the entrained solid particulates no longer varies with freeboard height. This height of the freeboard needs to be sufficiently high to assure essentially complete combustion of the solid carbon containing particulate material entrained from the fluidized bed.
In providing cost-effective fluidized bed combustors, especially those operating at high fluidization velocities or those of relatively large diameters, the excessive freeboard height required to provide an effective TDH has been found to be unsuitable since it contributes significantly to the high construction and maintenance costs of the fluidized bed combustors.
A recent development in fluidized bed combustion systems has provided a successful approach for decreasing the freeboard height. This promising approach used is a vortiginous fluidized bed combustion arrangement wherein vortex-generating mechanisms in the freeboard area are utilized to increase the turbulence in the freeboard through the formation of vortices in the freeboard. Normally, these vortices are formed by injecting secondary air into the freeboard through tangentially disposed injectors so as to, in effect, create a cyclone to provide centrifugal forces which act upon the entrained solid particulates in the freeboard area for moving the particulates against the inner walls of the combustor defining the freeboard zone. These solid particulates after impinging upon the walls, slide or descend down the wall as in the hopper and dipleg of a cyclone. This cyclone-type mechanism was expected to facilitate the return of the solid particulate material to the fluidized bed to effectively recycle the unburned particulate material, ash, and other solids to the fluidized bed for completing the combustion of the solid carbon-bearing particulate materials much in the same manner as the combustor systems utilizing cyclones disposed externally of the combustor. The vortices generated within the freeboard also increased the residence time of the solid particulates in the freeboard area so as to enhance the combustion of the carbon-containing gases and solids within the freeboard. By using vortex generators in the freeboard, the overall height of the freeboard could be reduced and the cyclone-type mechanism designed externally of the combustor could be eliminated.
While the fluidized bed combustion systems using vortex-forming generators in the freeboard have provided significant contributions to the fluidized bed combustion art, especially with respect to reducing the height of the freeboard, there were still several drawbacks or deficiencies which considerably detracted from their use in fluidized bed combustors. For example, the vortex-generating mechanisms as known in the art provided for the tangential introduction of the high velocity stream or jets of the secondary combustion supporting medium or air into the freeboard area at locations contiguous to the inner wall of the freeboard zone. This technique of introducing the tangential vortex-forming air imposed a substantial diametrical size limitation on the combustor in that these tangentially introduced jets of air could only form vortices of relatively large diameters in the freeboard. The momentum imparted by the tangentially-induced air streams in a freeboard is limited by the size of the vortex so that in freeboards of relatively large diametrical cross-sections a centrally located volume or eye of the vortex is relatively static or dormant except for ascending gas-particle movement from the fluidized bed since this volume is not influenced by the vortex. This static or vortex-free area within the vertical central region of the freeboard becomes increasingly undesirable with increasing size due to the fact that greater portions of the gaseous and solid particulate material ascending from the fluidized bed can readily escape from the freeboard through the discharge stack at the top of the combustor. The size or diameter of this vortex-free area within the center of the vortex is influenced by the orientation and the velocity of the air streams introduced tangentially to form the vortex.
Another drawback in previously known vortex generator mechanisms is due to the orientation of the vortex generators within the freeboard area in that the tangentially oriented nozzles utilized to provide the jets of the vortex-forming fluid have the outlets thereof essentially flush, if not flush, with the inner walls of the combustor defining the freeboard. This position of these nozzles creates a high velocity stream the vortex-forming air along or closely contiguous to the wall area where the nozzle outlets are located. This high velocity stream of air in turn contacts and entrains a considerable portion of the solid particulates that have been previously driven to the wall by the centrifugal force from vortices generated at higher levels in the freeboard. The net effect of this "reentrainment" condition is that many sections of solid recycling loops of different particle densities will be formed. The residence time for each size range of these recaptured solid particulates is not proportional to the size range of the particulates in the freeboard and thereby presents an unsatisfactory condition in the freeboard.
Still another problem with respect to these nozzles being disposed at locations flush or nearly flush with the inner wall of the combustor section defining the vapor space or freeboard is that the high velocity streams of gas carrying the solid particulates captured from the wall tend to erode the inner wall of the combustor at an excessively high rate. Efforts to overcome this severe erosion problem include the use of various vertically oriented flanges or shoulders for interrupting vortex flow along the walls, or the use of expensive refractory materials in the construction of freeboard walls.
In view of the problems associated with the previously known vortex-producing generators, the apparent advantages gained by using the vortex generators have been considerably degraded by the problems attendant with their use.